The molecular components of neuronal calcium channels have not yet been identified, however protein phosphorylation has been shown to play a role in the function of calcium channels. Two neuronal phosphoproteins, P96 and P139, were studied in rat brain synaptosomes and it was proposed that their phosphorylation may play a role in neuronal calcium channel function. Activation of calcium channels by depolarization leads to the extremely rapid dephosphorylation of these proteins. Although dephosphorylation was dependent on external calcium, it was also dependent on the mechanism of calcium entry through calcium channels, rather than simply to an increase in intracellular calcium. Thus, agents which stimulated rises in intracellular calcium did not initiate dephosphorylation. In contrast, drugs which activate (BAY K 8644) or inhibit (nifedipine, verapamil) calcium channels specifically increased the phosphorylation of P96 and P139. The data supported a proposal that these phosphoproteins may be involved in calcium channel function, either as integral proteins or regulatory proteins.